The problem of mutual connection of two rotating shafts in a way that energy and rotations are transferred from one shaft to another is encountered in engineering industry practically from its origin. Mutual position of shafts may vary, and adequate solutions for mutual connection of shafts exist for their various positions.
One of the most common situations is a situation, where the axis of a driving (input) shaft intersects in one point the axis of a driven (output) shaft, the angle between both axes of shafts is variable during operation and shafts cannot move in direction from or to the centre of a joint. A typical example of such type of connection is a constant velocity joint in the front axle of a vehicle with front drive, which connects the shaft of the front wheel with a half-axle protruding from the vehicle gear.
The first usable type of this kind of connection of two shafts was apparently a joint of an Italian inventor, Girolamo Cardano, in the 16th century, and its enhancement by Robert Hook in the 17th century. Universal joints, based on ideas of these two early-modern period scientists, are used by now, especially in agricultural machines, trucks, etc. Disadvantage of such designed joints consists in fact that angular velocity at the output shaft differs from angular velocity at output cyclically during each rotation, and it depends on mutual angle of shafts. The condition of equal angular velocities with this solution can be met in a way that both joints placed one after another are used so that the angle of the input shaft and joint axis is still the same as the angle of joint axis and the output shaft. In practice, however, it is a problem to achieve equal angles, and therefore, such types of joints are used only in applications, where homokineticity is not a critical characteristic.
The most frequently used type of a constant velocity joint is at present most probably a joint of so-called Rzeppa-type or “Ball-Type CV joint” (where CV stands for constant velocity). Most present solutions are more or less based on U.S. Pat. No. 1,665,280 of 1927, U.S. Pat. No. 1,916,442 of 1929, U.S. Pat. No. 2,010,899 of 1933 or other patents of the same author.
The arrangement of this type of a joint is generally based on six balls, which roll in paths with semicircular cross section, formed at outer ball surface of the input shaft and lie in planes crossing the shaft axis. The output shaft has a similar grooves formed, but located in inner surface of ball cavity of the output shaft. Each ball touches both the groove on the inner shaft and the groove on the outer shaft. The very important part of this joint is a massive metal cage, which is to keep centres of balls in a homokinetic plane. At one moment each ball touches the path on the inner shaft, on the outer shaft, and some of faces of openings placed on the massive cage. The big disadvantage of this type of a joint is the fact that entire loading transferred by the joint is actually transferred only by contact between the ball and outer and inner rolling path. Considering the small amount of balls, the tension in contact places is very high, and its size significantly limits maximum transferred performance and life of the entire joint. Rolling of balls in paths has not only rolling nature, but each ball has to turn around the axis, which is perpendicular to the shaft axis. Slipping of ball toward paths occurs, from which friction results. Another place where friction arises is in a place of contact of the ball and the cage, where only sliding friction always arises. The rate of sliding friction in this type of a joint is rather high, and shows mainly in considerable production of heat with bigger angles of joint tilting. The cage is inevitable for this type. The joint would not be able to work without it. Its homokineticity depends on quality of the cage and quality of its placing in this joint. And whereas the cage has to have a certain clearance, the joint of this type is not 100 percent homokinetic then. Another big disadvantage of this type of a joint is its zero tolerance to axial forces. The joint is not able to transfer any axial loading. The production of the joint is demanding, as it requires special machines for production and grinding of rolling paths in the cavity of ball shape.
Apart from above mentioned types of joints a big amount of more or less homokinetic solutions have been developed, which did not make the right at all or are used very rarely. These include, for instance, Tracta-type joints, represented, for instance, by the document FR 652829, Bendix-Weiss-type joints, represented, for instance, by the document DE 1 800 012, and the like.